1. Field of the Invention
The present invention relates to a method and system for controlling an engine.
2. Description of the Background Art
Modern automotive engines have a controller and a combustion chamber. The controller causes the combustion chamber to operate alternately on a lean air/fuel mixture (oxygen excess) and a rich air/fuel mixture (oxygen deficiency). The exhaust gases resulting from combustion are supplied to a catalytic converter, which is provided, inter alia, for reducing the nitrogen oxides.
Internal combustion engines of this kind are disclosed in U.S. Pat. No. 5,437,153 issued Aug. 1, 1995 to Takeshima et al., and U.S. Pat. No. 6,289,672 B1 issued Sep. 18, 2001 to Katoh et al.
Researches have used various names to refer to a catalytic converter of the above-mentioned kind. For example, Takeshima et al. called it xe2x80x9ca NOx absorbent or trap.xe2x80x9d Katoh et al. called it xe2x80x9ca NOx occluding and reducing catalyst.xe2x80x9d In the following description, the term xe2x80x9ca NOx trapxe2x80x9d is herein used to mean a catalytic converter of the above kind.
A NOx trap utilizes alkali metal or alkaline earth metal in combination with platinum in order to store or occlude the nitrogen oxides when there is oxygen excess. When there is oxygen deficiency, the NOx trap releases the trapped nitrogen oxides. Under this operating condition called xe2x80x9cpurge modexe2x80x9d, the oxygen is withdrawn from the absorbed nitrogen oxides, and the hydrocarbons (HC) and the carbon monoxides (CO) generated by the combustion are all oxidized with this oxygen.
The NOx trap can, however, only absorbs a limited mass of nitrogen oxides. As a result, the NOx trap must be purged after a certain loading time in which it traps the nitrogen oxides. During the purging or xe2x80x9cNOx purge cycle,xe2x80x9d the NOx trap releases the nitrogen oxides so that it can be charged anew. If the NOx trap is purged too late, it is xe2x80x9cfilledxe2x80x9d and can no longer absorb the nitrogen oxides, allowing them to escape into the environment. If the NOx trap is purged too long, it is xe2x80x9cemptyxe2x80x9d and can no longer supply nitrogen oxides as a source of oxygen for oxidizing the hydrocarbons and carbon monoxides, allowing them to escape into the environment.
The charging and purging of the NOx trap must therefore be controlled. This is achieved by means of the oxygen inflow. During oxygen excess, the catalytic converter is charged with nitrogen oxides. During oxygen deficiency, the NOx trap is purged and releases nitrogen oxides. In the above-mentioned Takeshima et al., the controller changes over from the oxygen excess to the oxygen deficiency when estimate, in mass or amount, of the absorbed nitrogen oxides exceeds a threshold.
In Takeshima et al., the controller causes an increase in fuel injection time to make air/fuel mixture in the combustion chamber rich when the oxygen deficiency is requested. Takeshima et al. also discloses application to Diesel engine wherein the controller causes an injector to feed reducing agent, such as, gasoline, into the exhaust pipe before the catalytic converter when the oxygen deficiency is requested.
In the above-mentioned Katoh et al., the controller causes a secondary fuel injection in the expansion or exhaust stroke to provide the oxygen deficiency when the engine operates on varying of air/fuel ratios falling in a region of moderate lean air/fuel mixtures with air/fuel ratios less than 20.
JP-A 11-294145 discloses an internal combustion engine equipped with an injector for feeding reducing agent into the exhaust pipe and an exhaust throttle upstream of the injector for restricting flow of exhaust gas to minimize consumption of reducing agent. In JP-A 11-294145, a controller, in response to request for the oxygen deficiency, causes restriction of exhaust gas flow as well as injection of reducing agent into the exhaust pipe for a catalyst to release nitrogen oxides. To suppress a drop in available engine torque due to pumping loss caused by the restriction of exhaust gases, the controller causes an alteration of at least one engine operating parameter to increase engine torque.
In the prior art, the secondary injection is carried out in response to the oxygen deficiency request. Utilizing the secondary injection causes an increase in fuel consumption. Besides, the fuel used for the secondary injection does not contribute to combustion, resulting in waste of energy. In the above-mentioned JP-A 11-294145, in order to compensate for reducing effect of available engine torque due to pumping loss caused by restriction of exhaust gas flow, the controller causes an increase in fuel to be combusted. The amount of such increase in fuel is so determined as to compensate for the reduction in available engine torque only without any concern on possible alteration in composition of exhaust gases resulting from the combustion of increased fuel with the decreased excess air ratio (xcex). Although this control strategy has utilized alteration of engine operating parameter to compensate for the reducing effect of available engine torque, the prior art fails to teach the enhanced torque control of the present invention.
In the prior art, the air/fuel mixture is altered in response to oxygen deficiency request. In the before-mentioned Takeshima et al., engine operation on rich air/fuel mixture is accomplished in response to the oxygen deficiency request. Under this operating condition, the excess air ratio is or less than unity (xcex=1 or xcex less than 1). This control strategy fails to account for alteration of available engine torque due to such change in air/fuel mixture. Apparently, such alternation of available engine torque is regarded as a problem in the before-mentioned Katoh et al. To solve this problem, Katoh et al. teach changing over from the oxygen excess to the oxygen deficiency when the alternation of available engine torque is not noticeable to the operator. Accordingly, the prior art has not yet to teach the enhanced torque control of the present invention.
There is, therefore, a need to develop a control strategy for controlling an engine utilizing enhanced engine torque control operation.
It is an object of the present invention to provide a method and system for controlling an engine through enhanced management of various control parameters.
In carrying out the above object and other objects and features of the present invention, there is provided a method for controlling an engine. The method comprises establishing a correction coefficient to compensate for reducing effect of available engine torque in operating range of different excess air ratios that are lower than a predetermined value. The method also comprises determining an initial base desired in-cylinder air mass based on a requested engine torque, and determining a desired excess air ratio. The initial base desired in-cylinder air mass is adjusted with at least the desired excess air ratio and the torque correction coefficient to generate a desired in-cylinder air mass. The method also comprises determining a desired injected fuel mass based on the desired in-cylinder air mass for fuel injection to deliver the requested engine torque with the desired excess air ratio held accomplished.
A system is also provided for carrying out the method.